1. Field of the Invention
This invention relates to liquid crystalline compounds and mixtures.
2. Description of the State of the Art
In an electric field, the molecules of liquid crystalline compounds and mixtures which possess a positive anisotropy of the dielectric constants (i.e. .epsilon..sub..parallel. &gt;.epsilon..sub..perp.) are oriented with their longitudinal axes (i.e., largest dielectric constant) parallel to the field direction. .epsilon..sub..parallel. signifies the dielectric constant along the longitudinal axis of the molecule and .epsilon..sub..perp. signifies the dielectric constant perpendicular thereto.
This dielectric field effect is utilized in the interaction between the liquid crystalline molecules and guest molecules (guest-host interaction) described by J. H. Heilmeier and L. A. Zanoni [Applied Physics Letter 13, 91 (1968)]. Another application of the dielectric field effect is the electro-optical rotation cell discovered by M. Schadt and W. Helfrich [Applied Physics Letters 18 (1971)]. A further example is the Kerr cell described in Molecular Crystals and Liquid Crystals 17, 355 (1972).
The electro-optical rotation cell includes a condenser-like structure having transparent electrode plates, the dielectric of which is formed from nematic liquid crystal material with .epsilon..sub..parallel. &gt;.epsilon..sub..perp.. The longitudinal axes of the liquid crystal molecules are arranged in twisted or helical form between the plates in the fieldless state. The twisting structure is determined by the given wall orientation of the molecules. After applying an electric potential to the condenser plates, the molecules adjust themselves with their longitudinal axes in the field direction, i.e., perpendicular to the surface of the plates, so that linear polarized light no longer rotates in the dielectric (i.e. the liquid crystal is uniaxially perpendicular to the surface of the plates). After removing the electric potential, the molecules return to their prior orientation. This reversible effect on the molecules can be used to electrically control the optical transmissivity of the condenser. To achieve an optimal transition between these two orientations, the threshold potential of the compounds or mixtures can be adjusted to the driving potential of the rotation cell. The driving potential of such a "light rotation cell" is dependent on the battery potential and the control circuit used. It thus becomes desirable to utilize liquid crystalline mixtures having low threshold potentials.
Further, a mixture of nematic liquid crystals with positive anisotropy and cholesteric substances (or generally, soluble, optically active substances provided the total mixture remains liquid crystalline) undergoes a phase transition upon application of an electric field. This phase change effect is reversible and makes it possible to have high switching speeds of electro-optical devices which operate with such mixtures. By selecting the concentration of cholesteric additives in the liquid crystal mixture, one attempts to improve the electro-optical properties of rotational cells.
It also is known that liquid crystalline mixtures with low viscosities have short response times.
We have invented liquid crystalline compounds and mixtures which advantageously possess low viscosities, good chemical stability, large mesophase ranges and high UV-stability.